In general, a boring machine for perforating the ground is based on a technique of simply circulating a bit (Oscillating method), a technique of not only circulating a bit or a ball cutter but also pressurizing the same (Reverse Circulation Drilling method: ROC), and so on. According to the oscillating method, in a state in which a standard casing having a diameter of 800 to 3000 mm is clamped by a hydraulic chuck, boring is performed by oscillating a cylinder installed rotatably in a left-right direction. According to the ROC method, the ground is bored using a drive rod having a rotary bit or ball cutter installed at its end portion by rotating the bit or ball cutter. The oscillation method can cope with a soft ground condition, that is, excavation is properly carried out through soft ground such as soil. However, for a hard-boring operation, it is necessary to demolish rocks under the ground by dropping a large-sized hammer, requiring additional equipment such as a pile driver.
Meanwhile, in the RCD method, which is an advanced method compared to the oscillation method from the viewpoint of boring capacity, a soil layer is first dug using an oscillator or a rotator, both a soft rock layer and a hard rock layer are dug by rotating drill rod a specially designed bit attached to its end portion, and air-suctioning circulating water and cloven rocks through a drill rod pipe, followed by hoisting the rocks to the surface of the ground. The RCD method is essentially employed in large-diameter cast-in-place and top-down method for a foundation work.
Examples of an air hammer for performing excavation are disclosed in U.S. Pat. Nos. 3,941,196, 0,430,554, and 3,991,834.
Since the conventional air hammer is moved downward to performing a striking action in a state in which it is separated from a guide, a vibration may occur while moving up and down. In particular, the air is rapidly cooled at a discharge outlet of the chamber due to adiabatic expansion, causing cracks. In addition, since there is no change in the air for upward moving the piston at the top dead center when the air hammer is moved upward, the shock may become increased. In addition, since a reaction force may become increased when the air hammer collides with the bit unit, the striking force of the bit unit may not be uniform.
An example of a boring machine using a crane is disclosed in Korean Patent No. 10-0372049.